00:00About 2,500 years ago, a earthquake radically changed the course of one of the most illustrious rivers on the planet, the Ganges.
00:08It is in the nature of watercourses not to remain immutable, so that if you live near one of them, it could one day evaporate without leaving a trace or, on the contrary, submerge an entire city.
00:20In the case of the Ganges, it is an earthquake of colossal magnitude that caused this spectacular metamorphosis.
00:27Researchers estimate that such an event could happen again, which would be a catastrophe for the millions of people whose existence depends on it.
00:36Born in the form of modest streams in the Himalayas, the Ganges traverses a distance of more than 2,500 kilometers.
00:43Throughout its journey, it merges with other large watercourses, including the Brahmaputra and the Meghna, before all these waters overflow into the Gulf of Bengal.
00:54Of all the rivers in the world, the Ganges has the largest delta.
00:58Like many other rivers in the region, its path evolves slowly over the centuries.
01:03When a river modifies its course by eroding its banks, these transformations can extend for several decades.
01:09Scientists have shown that the river system of the Ganges-Brahmaputra had undergone several shifts over the last six millennia.
01:17By examining the satellite images of the Ganges Delta, researchers made an intriguing discovery.
01:23They identified a depression in an arc of a circle about 50 kilometers from the current bed of the river.
01:29This formation, which extended over several kilometers and reached more than 1,000 meters wide,
01:34seemed to correspond to an ancient main arm of the Ganges, which had disappeared for centuries.
01:39In 2018, a team of researchers went there to analyze the region and collect samples to deepen their investigations.
01:47On the way back, they saw an excavation where a worker was digging to build a pond.
01:52The next day, this cavity was to be filled with water.
01:55But by examining the displaced earth, scientists spotted an unusual element.
02:01We could observe vertical strata of light sand mixed with darker mud.
02:05These particular formations are witnesses of the past.
02:08They reveal that an ancient earthquake caused a sudden rise in sand and water from the depths,
02:14creating sand volcanoes frozen in time.
02:17By analyzing these deposits of sand and mud, researchers have established that these earthquakes occurred about 2,500 years ago,
02:24and that they were of remarkable intensity, reaching magnitudes between 7 and 8.
02:30Although the epicenter was located more than 180 kilometers from the main course,
02:35the power of the phenomenon was enough to change the course.
02:39Such a disaster could have affected nearly 140 million people.
02:43Located at the junction of tectonic plates, the Ganges-Brahmaputra system is frequently subjected to earthquakes.
02:49No imminent danger has been identified,
02:52but experts estimate that another large-scale earthquake could occur in the next centuries or millennia.
02:58Such an event would threaten up to 170 million people,
03:02equivalent to half the population of the United States.
03:05However, it remains impossible to accurately predict the date and intensity of a future earthquake.
03:12The study of the past movements of watercourses following an earthquake
03:15allows researchers to identify trends and anticipate, to a certain extent, similar events.
03:21This knowledge could be valuable for strengthening preparation and setting up prevention strategies
03:26before a major disaster occurs.
03:29The Ganges is not the only river exposed to this type of threat.
03:33The Yellow River, the Irrawaddy, as well as several American watercourses,
03:37such as the Klamath and the San Joaquin, are also affected by these risks.
03:42The powerful Mississippi River has also changed course several times over the last seven millennia.
03:48Today, a vast network of dams and dikes, known as the Water Regulation Structures,
03:54has been erected in the center of Louisiana to contain these overflows
03:58and prevent it from merging with a neighboring river, the Atchafalaya.
04:02However, experts believe that this device could not withstand large-scale floods.
04:09Thus, the Mississippi could again cross its bed and change course,
04:13which would have terrible consequences, especially in southern Louisiana.
04:18One of the most striking recent examples of a disaster caused by the sudden change in the course of a river
04:24is that of the Kosi in India.
04:26In August 2008, in the space of a few days, the river deviated nearly 100 kilometers,
04:33a considerable upheaval.
04:35If the state of the Bihar is regularly hit by floods,
04:39a phenomenon of such magnitude had not occurred for more than half a century.
04:44This sudden crash affected nearly 1,000 villages and 3 million inhabitants,
04:48forcing the evacuation of a million of them to safer areas.
04:52Many of these populations, having not experienced such floods for decades,
04:56were not prepared to face them, which aggravated material and human losses.
05:01For decades, scientists have been trying to understand the precise mechanisms
05:05that drive a river to change its course abruptly.
05:09A recent study highlights the interaction of two main factors
05:13at the origin of this phenomenon, known as avulsion.
05:17First of all, a preparation is put in place,
05:20similar to an accumulation of tension before a sudden break.
05:24Over time, the sediments, sand, earth and other materials transported by the current
05:30accumulate in certain places.
05:32This accumulation can either increase the flow of water compared to the surrounding lands,
05:37or accentuate the slope of an adjacent area.
05:40Then comes the trigger, the decisive element that precipitates the change of course.
05:45It can be an exceptional creek, an earthquake,
05:49or even a simple floating pile of wood obstructing the flow of water.
05:53Researchers have analyzed satellite images covering 170 cases of avulsions
05:57over the last 50 years.
05:59They found that these events occurred most often near floodgates
06:03and mountainous reliefs.
06:05In these areas, rivers have shown a 3 times higher probability of changing course
06:10than in the intermediate plains.
06:12In addition, by studying in detail 58 river channels using high-resolution maps,
06:17they measured the elevation of watercourses compared to neighboring lands.
06:21They also evaluated the inclination of a possible new route compared to the current course,
06:26and found that near the mountains, the massive accumulation of sediments
06:30ended up elevating the bed of the rivers, causing overflows.
06:34But near floodgates, especially in the deltas,
06:37a marked slope is necessary to allow the rivers to pierce the mud and open a new passage.
06:44These two mechanisms interact like a swing.
06:47If one intensifies, the other can be less pronounced while allowing an avulsion.
06:52In order to predict the movement of a river in the event of a change of course,
06:56researchers have developed an advanced algorithm
06:59capable of analyzing the slope of the terrain and the river flow.
07:02Tested on 10 cases of reported avulsions,
07:05it has systematically anticipated with precision the path adopted.
07:09Scientists hope that these advances will improve the protection of river populations
07:14against sudden floods.
07:16The changes in river courses have played a decisive role
07:19in the emergence of one of the first organized administrative systems.
07:23Scientists have found clues in Mesopotamia,
07:27whose name means the land between two rivers,
07:30in reference to the Tigris and the Euphrates.
07:33The societies that thrived there depended on these rivers for their water supply,
07:37their agriculture and their trade.
07:39In order to understand the impact of sudden changes in the river network on these ancient populations,
07:44researchers relied on two types of maps.
07:47Archaeological maps retrace the expansion of cities over time
07:52and inform us about the types of buildings built.
07:55Some buildings, such as monumental palaces or fortified enclosures,
07:59testify to the emergence of a centralized administration.
08:02The second type of map allows us to reconstruct the ancient Tigris and Euphrates rivers,
08:07whose course has varied,
08:09sometimes slowly by describing a sinuous path,
08:12this is the meandering,
08:14sometimes brutally and spectacularly,
08:16this is the avulsion.
08:18Around 5000 BC,
08:21the climate of Mesopotamia gradually dried up,
08:24complicating agricultural practices.
08:26To survive,
08:27farmers had to settle near the rivers
08:30in order to take part in seasonal crops
08:33or to dig irrigation canals
08:35allowing water to flow to their crops.
08:37The vital importance of agriculture
08:39favored the growth of villages
08:41and cities along the rivers,
08:43thus guaranteeing the populations a constant access to their resources.
08:47But when a river suddenly changed course,
08:50the populations did not give up.
08:52They organized themselves to dig large canals
08:55to restore irrigation
08:57by capturing water from the new river bed.
08:59The emergence of infrastructure
09:01and administrative buildings
09:03testifies to their ability to organize collectively
09:06to meet all these challenges.
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